Vibration isolation is a classical problem in the mechanical engineering. Many machines, for example, cars, trains, heavy machinery, landing gears of airplanes, space landers, etc., require a vibration isolation system. The purpose of vibration isolation is to reduce the transmission of external disturbance to the sensitive parts of the system. A suspension, consisting of a spring and a damping element, may reduce the response of the sensitive parts of the system to the external disturbance, thus achieving the purpose of vibration isolation. Isolation systems are usually designed to attenuate either shock or persistent harmonic excitations.
People have been committed to the design and application research of passive vibration isolation systems for a long time. However, researchers have found that conventional passive vibration isolation systems are unable to harmonize the conflict between the resonant response and the high-frequency attenuation, thus the further improvement of the performance of the passive vibration isolation systems is restricted. To solve this problem, Karnopp and Crosby have proposed an ideal skyhook damping that can attenuate the resonant response without increasing the high-frequency transmissibility (D. Karnopp, M. J. Crosby, R. A. Harwood. “Vibration Control Using Semi-Active Force Generators”, Journal of Engineering for Industry, 96(2):6-9-626, 1974). A viscous damper in the vibration isolation system of the ideal skyhook damping is required to be connected to an inertial reference frame. However, in many practical applications, it is impossible that one end of a damper is connected to the isolated mass while the other end thereof is connected to an inertial reference frame. A vehicle suspension system is an obvious example. FIG. 1 shows a simplified ideal-skyhook damping vehicle suspension system. FIG. 2 shows an equivalent mechanical network of FIG. 1. One terminal of the isolated mass m2 is the center of mass, while the other terminal thereof is a fixed point in the inertial reference frame. For a system standing still relative to the inertial reference frame, the inertial reference frame becomes a common end of the damper csky and the isolated mass m2. Therefore, the damper csky may span and be connected in parallel to the isolated mass m2 via the inertial reference frame to absorb the vibration energy of the mass m2 and to suppress the resonance of the mass m2. However, for a system moving relative to the inertial reference frame, for example, a vehicle suspension, the damper csky is unable to span the isolated mass m2 without the inertial reference frame as a natural common end. This is the root cause why people think that an ideal skyhook damping cannot be realized passively.
To achieve the vibration isolation effect of the ideal skyhook damping, a replaceable implementation way is employed to realize the skyhook damping, including active and semi-active implementation ways. In the active implementation way, a sensor, an actuator and electronic control technology are employed to realize the skyhook damping (C. R. Fuller, S. J. Elliott, P. A. Nelson. “Active Control of Vibration”, Academic Press, New York,1996). In the semi-active implementation way, an electronically-controlled damping adjustment method is employed to realize the skyhook damping (S. Rakheja, “Vibration and Shock Isolation Performance of a Semi-Active ‘on-off’ Damper”, Journal of Vibration, Acoustics, Stress, and Reliability in Design, 107(4):398-403, 1985). Although the active and semi-active implementation ways can generate the expected effects in theory, the active and semi-active vibration isolation systems require external energy input, and have complex structure and poorer reliability than a passive vibration isolation system. Furthermore, during the vibration isolation, both an active vibration isolation system and a semi-active vibration isolation system will have three links, including the measurement by a sensor, the calculation by a controller and the execution by an actuation mechanism. There are many intermediate links. Furthermore, the errors and time-lag of the measurement by the sensor, the calculation by the controller and the actuation mechanism seriously affect the real-time performance and effectiveness of control, thus making the actual vibration isolation effect of the active and semi-active vibration isolation systems difficult to reach the expected effect in theory.
U.S. Pat. No. 6,315,094B1 disclosed a passive skyhook vibration isolation system, comprising a main vibration system and a dynamic vibration absorber with damping. In the main vibration system, a spring and a damper support a main mass. The dynamic vibration absorber with damping is attached onto the main mass of the main vibration system. The vibration of the main mass is suppressed by adjusting the parameters of the dynamic vibration absorber. In such a passive skyhook vibration isolation system, there is an irreconcilable conflict between the mass of the vibrator of the vibration absorber and the amplitude of the vibrator. According to the principle that the natural frequency of the vibration absorber is the same to that of the main vibration system, on one hand, if the amplitude of the vibrator is to be reduced, the stiffness of the spring of the vibration absorber is to be enhanced, and the mass of the vibrator is to be increased correspondingly. As a result, the mass attached onto the main mass will be increased certainly. Taking a car suspension system as example, the mass attached onto the car body will be 69 kg even though the minimum percentage of the mass of the vibrator to the main mass in this patent is 5%, given the mass of the car body is 1380 kg. Apparently, the kerb mass of the car increases. On the other hand, if the mass of the vibrator is to be reduced, the stiffness of the spring of the vibration absorber is to be reduced, thus the amplitude of the vibrator increases. Apparently, it is disadvantageous to the arrangement of the vibration absorber.
In conclusion, it may be found that there is an urgent demand for a passive skyhook and groundhook damping vibration isolation system, in order to overcome the shortcomings of the need of external energy input, complex structure, and poor reliability and real-time performance in active and semi-active implementation methods, simultaneously avoiding the problem of the conflict between the mass of a vibrator and the amplitude of the vibrator when a dynamic vibration absorber with damping is applied, harmonize the conflict between the resonant response and the high-frequency attenuation, and to suppress the resonance of the isolated mass without increasing the high-frequency transmissibility.